'utlist': linked list macros for C structures
=============================================
Troy D. Hanson <thanson@users.sourceforge.net>
v1.0, May 2009

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include::topnav_utlist.txt[]

Introduction
------------
include::toc.txt[]

A set of general-purpose 'linked list' macros for C structures are included with
uthash in `src/utlist.h`.  To use these macros in your own C program, just
copy `utlist.h` into your source directory and use it in your programs.

  #include "utlist.h"

These macros support the basic linked list operations: adding and deleting
elements, sorting them and iterating over them.

Download
~~~~~~~~
To download the `utlist.h` header file, follow the link to download on the
http://uthash.sourceforge.net[uthash home page]. Untar the package, and
look in the `src/` directory for `utlist.h`.

  tar -xjf uthash-1.6.tar.bz2
  cd uthash-1.6/src
  cp utlist.h <destdir>

BSD licensed
~~~~~~~~~~~~
This software is made available under the 
link:license.html[revised BSD license]. 
It is free and open source. 

Platforms
~~~~~~~~~
The 'utlist' macros have been tested on:

 * Linux, 
 * Mac OS X, and 
 * Windows, using Cygwin or MinGW.

Using utlist
------------

Types of lists
~~~~~~~~~~~~~~
Three types of linked lists are supported:

- *singly-linked* lists,
- *doubly-linked* lists, and
- *circular, doubly-linked* lists

Efficiency
^^^^^^^^^^
For all types of lists, prepending elements and deleting elements are
constant-time operations. Appending to a singly-linked list is an 'O(n)'
operation but appending to a doubly-linked list is constant time using these
macros.  (This is because, in the utlist implementation of the doubly-linked
list, the head element's `prev` member points back to the list tail, even when
the list is non-circular). Sorting is an 'O(n log(n))' operation.

List elements
~~~~~~~~~~~~~
You can use any structure with these macros, as long as the structure 
contains a `next` pointer. If you want to make a doubly-linked list, 
the element also needs to have a `prev` pointer.

  typedef struct {
      char *name;
      struct element *prev; /* needed for a doubly-linked list only */
      struct element *next; /* needed for singly- or doubly-linked lists */
  } element;

You can name your structure anything. In the example above it is called `element`.
Within a particular list, all elements must be of the same type. 

List head
~~~~~~~~~
The list head is simply a pointer to your element structure. You can name it
anything. *It must be initialized to `NULL`*.

  element *head = NULL;

List operations
~~~~~~~~~~~~~~~
The lists support inserting or deleting elements, sorting the elements and 
iterating over them.

[width="90%",cols="10^m,10^m,10^m",grid="cols",options="header"]
|===============================================================================
|Singly-linked             | Doubly-linked              | Circular, doubly-linked
|LL_PREPEND(head,add);     | DL_PREPEND(head,add);      | CDL_PREPEND(head,add;
|LL_APPEND(head,add);      | DL_APPEND(head,add);       | 
|LL_DELETE(head,del);      | DL_DELETE(head,del);       | CDL_DELETE(head,del);
|LL_SORT(head,cmp);        | DL_SORT(head,cmp);         | CDL_SORT(head,cmp);
|LL_FOREACH(head,elt) {...}| DL_FOREACH(head,elt) {...} | CDL_FOREACH(head,elt) {...}
|===============================================================================

'Prepend' means to insert an element in front of the existing list head (if any),
changing the list head to the new element. 'Append' means to add an element at the
end of the list, so it becomes the new tail element.  

The 'sort' operation never moves the elements in memory; rather it only adjusts
the list order by altering the `prev` and `next` pointers in each element. Also
the sort operation can change the list head to point to a new element.

The 'foreach' operation is for easy iteration over the list from the head to the
tail. A usage example is shown below. You can of course just use the `prev` and
`next` pointers directly instead of using the 'foreach' macros.

The parameters shown in the table above are explained here:

head::
  The list head (a pointer to your list element structure).
add::
  A pointer to the list element structure you are adding to the list.
del::
  A pointer to the list element structure you are deleting from the list.
elt::
  A pointer that will be assigned to each list element in succession (see
  example).
cmp::
  pointer to comparison function which accepts two arguments-- these are
  pointers to two element structures to be compared. The comparison function
  must return an `int` that is negative, zero, or positive, which specifies
  whether the first item should sort before, equal to, or after the second item,
  respectively. (In other words, the same convention that is used by `strcmp`).

Example
~~~~~~~
This example program reads names from a text file (one name per line), and
appends each name to a doubly-linked list. Then it sorts and prints them.

.A doubly-linked list
--------------------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "utlist.h"

#define BUFLEN 20

typedef struct el {
    char bname[BUFLEN];
    struct el *next, *prev;
} el;

int namecmp(el *a, el *b) {
    return strcmp(a->bname,b->bname);
}

el *head = NULL; /* important- initialize to NULL! */

int main(int argc, char *argv[]) {
    el *name, *tmp;

    char linebuf[BUFLEN];
    FILE *file;

    if ( (file = fopen( "test11.dat", "r" )) == NULL ) {
        perror("can't open: "); 
        exit(-1);
    }

    while (fgets(linebuf,BUFLEN,file) != NULL) {
        if ( (name = (el*)malloc(sizeof(el))) == NULL) exit(-1);
        strncpy(name->bname,linebuf,BUFLEN);
        DL_APPEND(head, name);
    }
    DL_SORT(head, namecmp);
    DL_FOREACH(head,tmp) printf("%s", tmp->bname);

    fclose(file);

    return 0;
}
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